Frame Structure and Modulation
The air interface of GSM mobile communication systems (GSM=Global System for Mobile Communications) is based on the TDMA technique (TDMA=Time Division Multiple Access), according to which a number of simultaneous calls may be transmitted on a same radio frequency. Calls transmitted on the same radio frequency are separated from each other in the time domain, i.e. the time on the radio path is divided into slices, i.e. time-slots, and each time-slot may be used for transmitting a separate call. This time-division forms a frame structure, which determines how many calls may be transmitted on one radio channel in the GSM system, it is possible to transmit up to eight calls in one frame structure on a full-rate speech channel. The frame structure of the GSM system is disclosed in M. Mouly & M. -B. Pautet, The GSM System for Mobile Communications, 1992, Mouly & Pautet, Palaiseau, France (in the following referred to as M & P), especially FIG. 4.15.
The properties of a transmitter of a GSM base station are described in two GSM specifications: GSM 05.04, January 1991, Recommendation GSM 05.04: Modulation, ETSI/PT 12 (3 pages in total), and GSM 05.05, March 1991, GSM Recommendation 05.05: Radio Transmission and Reception, ETSI/PT 12 (19 pages in total). Briefly, the above-mentioned recommendations make the following three demands for a transmitter: the broadest allowable frequency band (GSM 05.05 ANNEX 1), the maximum allowable phase error in modulated data (GSM 05.05, section 4.6), and power adjustment control for modulated data during a time-slot (GSM 05.05 ANNEX 2).
GSM recommendations define a TDMA frame to be used in GSM mobile communication systems, the time of which frame is divided into eight different time-slots. The maximum number of calls on one radio channel is thus eight when full-rate coding is employed. Naturally, it is also possible to use other rates, such as half-rate coding. The length of each time-slot is 156.25 bits, or they may alternatively be arranged so that the first and the fourth time-slot are longer, i.e. 157 bits in length, the other ones being 156 bits in length. In such a case, the lengths of the time-slots are in succession as follows: 157, 156, 156, 156, 157, 156, 156, 156 bits. This succession is disclosed in GSM recommendation GSM 05.10, January 1991: Radio Sub-system Synchronization, ETSI/PT 12 (6 pages in total), section 5.7. In that case, the length of each bit is 3.69 .mu.s.
Each time-slot contains 148 bits of data to be transmitted, and a guard-time of 8.25 bits. The binary data, i.e. the 148 bits transmitted during each time-slot contain a training sequence of 26 bits. This sequence of bits is known both by the transmitter and by the receiver, and its location is always the same. By means of a training sequence, it is possible to determine the location of the start and the end of a time-slot when the location of a training sequence of the transmission is detected within a time-slot. The training sequence is also described in M & P, in particular in chapter 4.3.1.1 and table 4.3. The training sequence may thus be used for synchronizing the transmitter and the receiver.
The guard-time, i.e. the 8.25 bits remaining from said 148 bits is intended for power adjustment carried out between different time-slots (M & P: FIG. 4.23). Half of the bits are located at the beginning of the time-slot, and another half at the end of the time-slot. Power adjustment is carried out so that at the end of data sent in the preceding time-slot and coming from a GMSK modulator, the Dower is dropped along a falling cosine curve close to zero, and, at the beginning of a new time-slot, the power is increased along a rising cosine curve to a desired power level before sending the actual 148 bits, i.e. a normal burst.
The guard times at the beginning and at the end of the time-slots may be used for power adjustment, and data is neither transmitted nor received during them. The guard times are located between two time-slots. The location of the beginning and the end of a time-slot, and that of the guard times are determined by means of the above-mentioned training sequence in the data to be transmitted.
The binary data to be transmitted is modulated by means of a GMSK modulator in accordance with the GSM recommendation 05.04, see also M & P: section 4.3.4. The GMSK modulator used is a Gaussian Minimum Shift Keying modulator